The a-Si:H thin film has been well studied in recent years, and its use in not only a solar cell and a photosensitive drum but also a scanning circuit of an image reading device, a driving circuit for image display devices, etc. has been considered.
The a-Si:H thin film is usually produced by glow discharge, sputtering, CVD (chemical vapor deposition), photo CVD (photoinduced chemical vapor deposition), etc., but with these methods, the speed of forming the thin film was as slow as several .ANG./sec or below. To increase the film-forming speed, the vapor deposition and ion plating techniques have also been studied. But with these methods, the quality of the resulting film which relates to photoelectrical properties becomes poor and the film cannot be used as such in the above-mentioned devices. Accordingly, at present, the film-forming speed is sacrificed, and by the glow discharge of SiH.sub.4, an a-Si:H thin film is prepared at a speed of not more than 5 .ANG./sec.
A method of improving photoelectrical properties by increasing the RF power is disclosed in the specification of Japanese Laid-Open Patent Publication No. 186319/1982. This method is to form a thin film of polycrystalline silicon, and the electric power required for crystallization far exceeds the energy level employed in the present invention and should be 800 KJ/g-SiH.sub.4 at the lowest. Accordingly, this method is disadvantageous in respect of energy, and for crystallization, the speed of forming the thin film must be maintained low. Hence, an increase in the film-forming speed cannot possibly be achieved.
One known application of the a-Si:H thin film is in an a-Si:H cell. The a-Si:H cell has recently been studied everywhere, and an advance has been noted in the efficiency achieved. However, in order to increase the efficiency in the prior art, the speed of forming the amorphous silicon active layer must be maintained at as low as not more than 5 .ANG.. If the film-forming speed is increased in the formation of the a-Si:H cell from SiH.sub.4, the efficiency of photoelectric conversion will be extremely reduced and reach only 1 to 2% at the highest.
The present inventors previously found that the aforesaid photoelectric conversion efficiency can be increased when an active thin film layer of amorphous silicon is formed by using Si.sub.N H.sub.2n+2 (n=2 or 3) instead of SiH.sub.4 as a gas capable of forming a thin silicon layer in the production of a hydrogenated amorphous silicon solar cell composed of a substrate having a first electrode and, formed thereon in the order to be mentioned, an amorphous silicon thin film layer of a first type electrical conductor, an essentially intrinsic amorphous silicon thin film layer, an amorphous silicon thin film layer of a second type electrical conductor and a second electrode (Japanese Patent Application No. 4978/1982). According to this method, the speed of forming the amorphous silicon thin film layer is in the range of 5-10 .ANG./sec., and the efficiency decreases if the film forming speed is increased.
In the prior techniques described above, if an a-Si:H cell is formed by increasing the speed of forming at least the amorphous silicon thin film layer to 6 .ANG./sec or more, especially to 15 .ANG./sec or more, the photoelectric conversion efficiency of the cell is decreased to 2% or less, and the cell loses its utilitarian value.
The present inventors have made extensive investigations in order to solve the aforesaid problems of the prior methods, and have found that the speed of forming the a-Si:H film can be increased by controlling the amount of the supplied energy in relation to the speed of forming the thin film, and that when this technique is applied to the production of a solar cell, the solar cell can be produced at a high speed while maintaining a high photoelectric conversion efficiency. This discovery has led to the present invention.